The present invention relates to a process for manufacturing sintered compacts from metal powder having predominant constituent of Al.
Aluminum-base metal sintered compacts have found many practical uses for various machine parts, since they are light in weight, reveal higher strength and show high anti-corrosive property. In manufacturing these aluminum-base sintered compacts, it is required to sinter the metal powder of aluminum or of aluminum alloy at higher temperature such as 500.degree..about.600.degree. C. If the surface oxidation of the metal particles proceeds during the period of heating up to the sintering temperature or the period of sintering, the binding force between the particles next to each other becomes weak and the strength of the sintered compact decreases. In order to prevent such surface oxidation of the metal particles, it has heretofore been practised to carry out the sintering in a non-oxidizing environment such as in an inert gas or in vacuum. However, in preparing the sintering atmosphere to be non-oxidizing, the costs for the manufacturing equipments and for the operation become considerably high, so that it will be impossible to provide sintered products at lower price. It has thus recently been demanded earnestly to develop and establish a process for manufacturing aluminum-base alloy sintered compacts in which the sintering can be accomplished in the air without necessitating the preparation of special sintering atmosphere.
As to the process for manufacturing sintered compact of aluminum-base alloy in which the sintering is carried out in the air, a process has already been proposed by S. Storchheim according to U.S. Pat. No. 3,687,657, in which, using a powder mixture prepared by admixing a small amount of magnesium powder and/or zinc powder to aluminum powder, a low melting eutectic of aluminum with magnesium or with zinc is formed during the course of heating of the powder mixture up to the sintering temperature and thus the surface of particle of aluminum powder is covered with this eutectic, whereby the oxidation of surface of aluminum particles during the sintering is prevented.
While it is in fact possible by this process to carry out the sintering in the air, it has been recognized according to the experiments conducted by us, that the sintered compacts obtained by this process revealed only low age hardening effect, as the experiments showed that sintered compacts thus obtained exhibited, after they had been subjected to solution heat treatment and subsequent quenching followed by an artificial age hardening, only low tensile strength of about 20 kg/mm.sup.2 or so. It was furthermore disclosed by our experiments, that an enough mechanical strength was never attainable by said proposed process, so long as extremely fine starting powder was not employed. Thus, for example, it was by all means required, to use for the aluminum powder those which contain 35.about.95% or more of a fine particle fraction with particle size passing completely 350-mesh of Tyler standard sieve, necessitating at the same time to adopt other additive metal powder also in size all passing through the 350-mesh sieve. Thus, in the process according to the proposal of S. Storchheim, it is not only of disadvantage, that the starting powder is costly in price, but it is also problematic, that the worse flowability of the powder due to its extremely fine particle size will result in a retardation of velocity for charging the die in the compacting procedure, whereby the productivity is decreased.
In order to increase the mechanical strengths of the sintered compacts of aluminum-base alloys, a possibility of addition of an element contributive to the precipitation hardening of the aluminum matrix, such as, silicon, copper and so on, may be expected as a countermeasure. Among these additive elements, silicon may particularly be assumed to contribute greatly to the precipitation hardening. As for the practical method of addition of silicon, admixing of powdery simple substance of silicon to the starting powder may be considered at the first place. According to the experiments performed by us, however, it has been revealed, that the rate of diffusion of silicon is so low that the silicon particles are retained as such in the sintered body even after a considerable duration of sintering and thus a contribution by the precipitation hardening cannot be expected but, on the contrary, even a decrease in the strength may be observed.
As another method of addition of silicon, there may be considered to admix a powder of Al-Si alloy to the starting powder. In general, however, the compactibility in compacting the metal powder is worse for powder of alloy having higher silicon content. Therefore, if a large amount of Al-Si alloy powder having higher content of silicon is added, the green compact will not sufficiently be densified during the compacting procedure, so that there may occur a danger of progress of internal oxidation upon sintering in the air.